Two-phase flows at small scale - Application to proton exchange membrane fuel cell cooling (PEMFC)

This study is devoted to boiling flows in mini-channels of small dimensions. The objective was the enhancement of the thermal performance of PEM fuel cells. Confinement is nowadays one of the key parameters when considering a fuel cell. Indeed, the amount of heat flux to be removed from the fuel cell increases when its dimensions are decreased. As a consequence, thorough investigations of heat transfer in fuel cells are required. In our study, Forane 365 HX was selected to be used as the working fluid for the purpose of cooling. In comparison with single-phase flows, an important amount of energy can be extracted in forced boiling conditions while keeping a moderate wall temperature gradient. Thus, in order to properly design a heat exchanger in forced boiling conditions, it is important to understand the influence of the dimensions of one single mini-channel on the pressure drop across the channel and the heat transfer coefficients. Consequently, we investigated the flow and associated heat transfer in four rectangular mini-channels of hydraulic diameter ranging from 0.727 mm to 1.45 mm. We especially studied the influence of the aspect ratio by considering two mini-channels of same hydraulic diameter and different aspect ratios. We found that the heat flux necessary to the onset of nucleate boiling or the critical heat flux are dependent on the geometric dimensions of the mini-channel in our experiments. For the saturated boiling flow conditions, the pressure drop is also related to the channel cross-section area whereas the heat transfer coefficients seem to be nearly independent of the mini-channel under study. Simultaneously to these measurements, a high-speed video camera was used for visualizing the boiling flow. We observed that the mini-channel cross-section has a significant influence on the flow pattern transitions. Most of our experiments were performed for a working pressure of 70 kPa (relatively to atmospheric pressure) whereas the mass flow rate was in the range [100- 800 kg.m^-2.s^-1]. The heat flux removed by the boiling flow reached values as large as 250 kW.m^-2 for the critical heat flux conditions. (author)